Lyric discussion by fish2o 

Cover art for Action Man lyrics by Widespread Panic

Mahuba was indeed Man O' War's dam (mother). The "Willie" that is referenced is Will Harbut, who was Man O' War's groom who said that Man O War was the "mostest horse".

I hear "???? gonna take the highest prize" as "War Admiral takes the highest prize". War Admiral, who was Man O War's son, won the Triple Crown in 1937, d was one of Man O War's most successful offspring.